This invention relates to permanent magnets, Rxe2x80x94TMxe2x80x94B based permanent magnets, where R is a rare earth element embracing Y and TM is a transition metal, and, more particularly, to a starting material thereof, an intermediate product thereof and an ultimate product thereof.
Additionally, this invention relates to rare-earth magnetic powders for bonded magnets and a manufacturing method thereof.
The mechanism used for generating the coercivity in permanent magnets currently under use may be enumerated by single magnetic domain particle type, nucleation type and pinning type mechanisms. Of these, the nucleation type coercivity generating mechanism has been introduced in order to account for generation of large coercivity in a sintered magnet having a crystal grain size not less than the single magnetic domain particle size, and is based on the theory that facility of nucleation of an demagnetizing field in the vicinity of the crystal grain boundary determines the coercivity of the crystal grain in question. This type of the magnet has peculiar magnetization properties that, while saturation of magnetization in the initial process of magnetization occurs at a lower impressed magnetic field, a magnetic field not less than the saturation magnetization needs to be applied to obtain sufficient coercivity. It may be presumed that the high magnetic field can drive off any demagnetizing field left in the crystal grain completely by a high magnetic field thus producing high coercivity. Examples of the magnet having the nucleation type coercivity generating mechanism include SmCo5-based or Ndxe2x80x94Fexe2x80x94B-based sintered magnets.
The Rxe2x80x94TMxe2x80x94B based permanent magnet has superior magnetic properties, and is finding a wide field of usages. There are a variety of manufacturing methods for the Rxe2x80x94TMxe2x80x94B based permanent magnet, the most representative one being a sintering method and a rapid solidification method. The sintering method, as disclosed in Japanese Laying-Open Patent Kokai JP-A-59-46008, is a method consisting in pulverizing an ingot of a specified composition to fine powders of single crystals with a mean particle size of several xcexcm, consolidating the powders to an optional shape under magnetic orientation in a magnetic field, and sintering the green compact to a bulk magnet. The rapid solidification method, disclosed in Japanese Patent Kokai JP-A-60-9852, is a method consisting in rapidly solidifying an alloy of a specified composition by a method such as roll quenching method to an amorphous state followed by heat treatment to precipitate fine crystal grains. The magnet alloy obtained by the rapid solidification method is usually powdered and are routinely mixed with a resin and molded to produce bonded magnets.
Rare earth magnetic powders having the coercivity generating mechanism of the pinning type, such as Sm2Co17, can be processed into magnetic powders suitable for bonded magnets simply by pulverizing a molten ingot of a pre-set composition. On the other hand, in rare earth magnetic powders having the coercivity generating mechanism of the nucleation type, practically useful coercivity is not produced unless the crystal grain size of the powdered particles is set so as not to be larger than the single magnetic domain particle size. Thus, as a manufacturing method in which the Nd2Fe14B crystal grain size in the powdered particles is less than the single magnetic domain particle size, there are currently used a rapid solidification method and a HDDR (hydrogenation-decomposition- dehydrogenation- recombination) method.
The present inventors have found that the conventional techniques concerning the above-mentioned nucleation type magnet has the following disadvantages. That is, while it has been predicted that, in the conventional techniques, the coercivity of the nucleation type magnet is governed by nucleation of the demagnetizing field, sufficient information has not been acquired as to specified means for suppressing nucleation of the demagnetizing field to improve the coercivity. For instance, while it has been known that the presence of the Nd-rich grain boundary phase operates to improve the coercivity in the Ndxe2x80x94Fexe2x80x94B based sintered magnet, its detailed mechanism has not been clarified.
In the above-described conventional techniques, sample preparation and evaluation are repeatedly carried out to optimize various conditions of the manufacturing process of the magnet to improve the magnetic properties of the magnet by an empirical route. However, with such an empirical method, it is difficult to achieve drastically improved magnetic properties. Moreover, if plural permanent magnets of different compositions are produced, the sample preparation and evaluation of the different magnets need to be repeatedly carried out for the respective magnets.
In the above-described a manufacturing method in which the Nd2Fe14B crystal grain size in the powdered particles is less than the single magnetic domain particle size, the rapid solidification method and the HDDR method suffer from the defect that the investment costs for production equipment are high and the manufacturing conditions are severe to raise the cost.
It is an object of the present invention to provide a guide or key for the designing of high magnetic performance.
It is another object of the present invention to provide a guideline for the designing of the Rxe2x80x94TMxe2x80x94B based permanent magnet having high magnetic performance.
It is a further object of the present invention to provide rare-earth magnetic powders for bonded magnets having high magnetic properties, and which can be manufactured inexpensively, and a manufacturing method thereof.
Heretofore, the structure of an interface governing the magnetic properties of a magnet, in particular its coercivity, between the major phase and the grain boundary phase, has not been clarified. In the present specification, the xe2x80x9cmajor phasexe2x80x9d means the xe2x80x9cphase exhibiting the ferromagnetismxe2x80x9d. The major phase desirably accounts for not less than one half of the entire phase. Thus, in the conventional technique, various conditions of the magnet manufacturing process are optimized for empirically improving the magnetic properties of the magnet. This empirical technique is not only time-consuming and costly but also is encountered with limitations in further improving the magnetic properties.
The present inventors have conducted researches into the fundamental problem of what should be the ideal interface structure, without relying upon the empirical technique, and found that, in a variety of magnetic materials exhibiting nucleation type coercivity generating mechanism, the ease with which nucleation occurs depends on the magnitude of the magnetocrystalline anisotropy in the vicinity of the outermost shell of the magnetic phase, and that, by controlling the magnitude of the anisotropy constant K1 in the vicinity of the outermost shell to be at least equal or larger than that in an interior region, the nucleation can be suppressed to improve coercivity of the magnet. This finding has led to completion of the present invention.
The First Group of the Present Invention
In a first aspect of the first group of the present invention, the ferromagnetic phase is matched with the grain boundary phase. In its second aspect of the first group, the atomic arrangement (orientation) is regular on both sides of an interface between the ferromagnetic phase and the grain boundary phase. In its third aspect of the first group, the grain boundary phase has a crystal type, a plane index and azimuthal index (crystal orientation) matched to the ferromagnetic phase. In its fourth aspect of the first group, the magnetocrystalline anisotropy at a lattice point of said ferromagnetic phase neighboring to the interface with the grain boundary phase is not less than one-half the magnetocrystalline anisotropy at the lattice point interior of said ferromagnetic phase.
In its fifth aspect of the first group, the magnetocrystalline anisotropy in the outermost shell of the ferromagnetic particles is not less than one-half that in the interior thereof. In its sixth aspect of the first group, the magnetocrystalline anisotropy in the outermost shell of the ferromagnetic crystal grains is higher than that in the interior thereof. In its seventh aspect of the first group, the magnetocrystalline anisotropy of the outer shell within five atomic layers from the outermost shell of the ferromagnetic crystal grains is higher than that in the interior thereof. In an eighth aspect of the first group, the magnetocrystalline anisotropy of the ferromagnetic crystal grains is displayed mainly by crystal fields arising from rare earth elements, and cations are located in the extending direction of the 4f electron cloud of rare earth element ions located at an outermost shell of the ferromagnetic crystal grains. In its ninth aspect of the first group, the cationic source is one or more of Be, Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Zr, Nb, Mo, Cd, In, Sn, Ba, Hf, Ta, Ir or Pb.
In a tenth aspect of the first group of the present invention, a cationic source is added to ferromagnetic particles exhibiting magnetocrystalline anisotropy mainly by the crystal field of rare earth elements, a crystal containing the cationic source is precipitated at least in a grain boundary portion neighboring to ferromagnetic grains and cations are located in a transverse direction of the extending direction of the 4f electron cloud of rare earth element ions located at an outermost shell of grains ferromagnetic particles. In its eleventh aspect of the first group, the composition, crystal type, plane index and azimuthal index of the grain boundary phase in the state of co-existence of both the ferromagnetic phase and the grain boundary phase, are set in accordance with the crystal structure of the ferromagnetic phase so that the ferromagnetic phase will match with the grain boundary phase.
The present invention has, in its first aspect of the second group, the following elements, namely a magnetic phase mainly composed of R2TM14B intermetallic compound having a tetragonal crystal structure (R: rare earth element including Y and TM: transition metal), and a grain boundary phase mainly composed of an Rxe2x80x94TM alloy, with the crystal structure of the grain boundary phase in the vicinity of the interface between the magnetic phase and the grain boundary phase being a face-centered cubic structure, with the magnetic phase and the grain boundary phase matching with each other. In its second aspect of the second group, in the R2TM14B intermetallic compound, the sum of Nd and/or Pr in R is not less than 50 at %, and TM is Fe and/or Co, with Fe in TM being at least 50, at %, and R in the Rxe2x80x94TM alloy being not less than 90 at %. In its third aspect of the second group, the crystallographic orientation in the vicinity of the interface between the magnetic phase and the grain boundary phase is represented by at least a set of expressions (A) to (C):
(001)magnetic phase//(110)grain boundary phase and [110]magnetic phase//[001]grain boundary phasexe2x80x83xe2x80x83(A)
(001)magnetic phase//(221)grain boundary phase and [110]magnetic phase//[111{overscore ( )}]grain boundary phasexe2x80x83xe2x80x83(B)
(001)magnetic phase//(111)grain boundary phase and [100]magnetic phase//[11{overscore ( )}]grain boundary phasexe2x80x83xe2x80x83(C)
and wherein the angle of orientation deviation is not larger than 5xc2x0.
In its fourth aspect of the second group, the permanent magnet is composed that
R is 8 to 30 at %;
B is 2 to 40 at %; with
the balance mainly being TM (particularly, Fe, Co).
In its fifth aspect of the second group, a magnetic phase has a crystal structure of a tetragonal structure and a grain boundary phase having a face-centered cubic crystal structure in the vicinity of an interface thereof with respect to the magnetic phase. The magnetic phase and the grain boundary phase are matched with each other interposed with an interface. In its sixth aspect of the second group, a source of an R2TM14B intermetallic compound exhibiting ferromagnetic properties (R: rare earth element embracing Y, and TM: transition metal) and an Rxe2x80x94TM alloy source are used as a starting material, and the R2TM14B tetragonal crystal phase is precipitated, while further an Rxe2x80x94TM face-centered cubic crystal phase is precipitated around the R2TM14B tetragonal phase to match the R2TM14B tetragonal phase and the Rxe2x80x94TM face-centered cubic crystal phase to elevate the magnetocrystalline anisotropy of the R2TM14B tetragonal phase in the vicinity of the matched (epitaxial) interface.
Taking an example of an Rxe2x80x94TMxe2x80x94B based permanent magnet, mainly composed of the major phase (ferromagnetic phase) composed of an R2TM14B intermetallic compound (preferably single crystal) and the grain boundary phase composed of a grain boundary phase composed of an Rxe2x80x94TM alloy, the principle in the second group of the present invention is explained. In a known manner, there exist in the Rxe2x80x94TMxe2x80x94B based permanent magnet a B-rich phase (R1+xcex1TM4B4), Rxe2x80x94TM meta-stable phase, oxides inevitably entrained in the process, and carbides, in addition to the above-mentioned major phase and the grain boundary phase. However, the effects of these phases on the magnetic properties of the permanent magnet are of subsidiary nature as compared to two phases of the major phase and the grain boundary phase.
The presence of the grain boundary phase is indispensable for the demonstration of practically useful coercivity. Generally the coercivity decreases as the R component in the magnet composition gets short, the R being required for forming the grain boundary phase. The reason is possibly that the two phases, namely the R2TM14B phase and the Rxe2x80x94TM phase cease to be able to co-exist in the equilibrium state due to shortage of the R component and that, in its stead, the ferromagnetic phase such as R2TM17 phase is precipitated in the grain boundary of the R2TM14B phase to form an origin of generation of the demagnetizing field (inverse magnetic domain) to produce inversion of magnetization easily to lead to a lowered coercivity. The compositional region in which the above-mentioned R2TM14B phase and the Rxe2x80x94TM phase coexist may be known from the Rxe2x80x94Fexe2x80x94B ternary equilibrium diagram.
For affording practically sufficient coercivity to the Rxe2x80x94TMxe2x80x94B based permanent magnet, prepared by the sintering method, it has been known necessary that the major phase as the ferromagnetic phase be contacted with the grain boundary phase at a smooth interface free of lattice defects, as has been clarified by microscopic observation of the interface over a transmission electron microscope. The reason is that, if there is a lattice defect in the interface, this lattice defect becomes the source of generation of the reverse magnetic domain to induce inversion of magnetization easily to lower the coercivity.
The present inventors have found that there exists the following problem in displaying superior magnetic properties proper to the Rxe2x80x94TMxe2x80x94B based permanent magnet of the above-mentioned prior art. That is, although the information on the composition range where there exists the Rxe2x80x94TM grain boundary phase or on the possible presence of the defects in the interface between the major phase and the grain boundary phase has been acquired in the prior art, there lacked the knowledge as to the crystal structure or the Rxe2x80x94TM grain boundary phase or the desirable relative orientation with respect to the major phase. Therefore, it has not been possible to control the microscopic structure of the Rxe2x80x94TMxe2x80x94B based permanent magnet having the specified composition to display superior magnetic properties. Instead, the various conditions of the magnet manufacturing process are optimized in the prior art with a view to empirically improving magnetic properties of the magnet.
That is, the magnetic properties of the magnet, in particular the structure of the interface between the major phase governing the coercivity and the grain boundary phase, were not known in the prior art. Thus, a variety of processing operations felt to vary the interface structure, such as heat treatment, are performed on the magnet to control the properties of the magnet, with the interface state remaining as a black box. Although this technique is not obstructive to the optimization of the manufacturing conditions of the magnets of various compositions, it is extremely difficult to improve the properties of the magnet further in the absence of the material development guideline as to what should be the ideal interface structure.
The present inventors have conducted microscopic analyses of the grain boundary phase of a variety of Rxe2x80x94TMxe2x80x94B based permanent magnet, using a transmission electron microscope (TEM), and found that, in the grain boundaries of all Rxe2x80x94TMxe2x80x94B based permanent magnets, there necessarily exists a grain boundary phase composed of a Rxe2x80x94TM alloy (generally, containing not less than 90 at % of R), and that superior magnetic properties can be realized when the crystal structure of the grain boundary phase in the vicinity of the interface relative to the major phase assumes a face-centered cubic structure.
The present inventors also conducted detailed scrutiny into the structure of the interface between the grain boundary phase of the Rxe2x80x94TMxe2x80x94B based permanent magnet having the Rxe2x80x94TM grain boundary phase of the above-mentioned face-centered cubic structure and the major phase (R2TM14B phase) by observation over a high resolution transmission electron microscope (HR-TEM) or a scanning tunnel microscope, and found that the magnetic properties are optimum when the microscopic structure of the permanent magnet is controlled so that the major phase and the grain boundary phase will have a specified relative crystallographic orientation in the vicinity of the interface to be matched with each other. The present invention has been brought to completion on the basis of this finding and our further perseverant researches.
The present invention has, in its first aspect of the third group, the following elements, namely a magnetic phase mainly composed of R2TM14B intermetallic compound having a tetragonal crystal structure (R: rare earth element embracing Y, and TM: transition metal), and a grain boundary phase mainly composed of an R3TM alloy, with the crystal structure of a portion of the grain boundary phase in the vicinity of the interface between the magnetic phase and the grain boundary phase being a rhombic structure, with the magnetic phase and the grain boundary phase matching with to each other. In its second aspect of the third group, in the R2TM14B intermetallic compound, the sum of Nd and/or Pr in R is not less than 50 at %, and TM is Fe and/or Co, with Fe in TM accounting for not less than 50 at %. In its third aspect of the third group, in the R2TM14B intermetallic compound, Fe in TM accounts for not less than 50 at % and Co in TM is not less than 0.1 at % and, in the R3TM intermetallic compound, Co in TM is not less than 90 at %. In its fourth aspect, the crystallographic orientation in the vicinity of the interface between the magnetic phase and the grain boundary phase is represented by at least a set of expressions (F) to (I):
(001)magnetic phase//(001)grain boundary phase and [110]magnetic phase//[110]grain boundary phasexe2x80x83xe2x80x83(F)
xe2x80x83(001)magnetic phase//(110)grain boundary phase and [110]magnetic phase//[001]grain boundary phasexe2x80x83xe2x80x83(G)
(001)magnetic phase//(221)grain boundary phase and [110]magnetic phase//[111{overscore ( )}]grain boundary phasexe2x80x83xe2x80x83(H)
(001)magnetic phase//(111)grain boundary phase and [100]magnetic phase//[11{overscore ( )}0]grain boundary phasexe2x80x83xe2x80x83(I)
with the angle of orientation deviation being not larger than 5xc2x0.
In its fifth aspect of the second group, the permanent magnet is composed that
R is 8 to 30 at %;
B is 2 to 40 at %;
Fe is 40 to 90 at %; and
Co is 50 or less.
In its sixth aspect of the third group, the crystal structure contains a magnetic phase having the crystal structure of a tetragonal system and a grain boundary phase having a crystal structure of a rhombic system in the vicinity of an interface to the magnetic layer. The magnetic phase is matched with the grain boundary phase interposed with the interface. In its seventh aspect of the third group, the present invention includes employing a source of an R2TM14B intermetallic compound exhibiting ferromagnetic properties (R: rare earth element embracing Y; TM: transition metals) and an Rxe2x80x94TM alloy source, as a starting material, precipitating an R2TM14B tetragonal crystal phase and precipitating the R3TM rhombic phase around said R2TM14B tetragonal crystal phase for matching the R3TM rhombic phase to the R2TM14B tetragonal crystal phase for elevating magnetocrystalline anisotropy of the R2TM14B tetragonal crystal phase in the vicinity of the matched interface.
Taking an example of an Rxe2x80x94TMxe2x80x94B based permanent magnet, mainly composed of the major phase (ferromagnetic phase) composed of an R2TM14B intermetallic compound (preferably single crystal) and the grain boundary phase composed of a grain boundary phase composed of an R3TM alloy, the principle in the third group of the present invention is explained. In a known manner, there exist in the Rxe2x80x94TMxe2x80x94B based permanent magnet a B-rich phase (R1+xcex1TM4B4), Rxe2x80x94TM meta-stable phase, and oxides, inevitably entrained in the process, and carbides, in addition to the above-mentioned major phase and the grain boundary phase. However, the influences of these phases on the magnetic properties of the permanent magnet are of subsidiary nature as compared to two phases of the major phase and the grain boundary phase.
In an Rxe2x80x94TMxe2x80x94B based permanent magnet, it is known that the Curie temperature is raised and corrosion resistance is improved by having Co contained in TM, such that it is a known technique to add a suitable amount of Co to the Rxe2x80x94TMxe2x80x94B based permanent magnet to this end. In addition to the above methods of processing the Rxe2x80x94TMxe2x80x94B based permanent magnet, there are a variety of known methods, such as mechanical alloying method, hot pressing method, hot rolling method and a HDDR method. However, all of the Rxe2x80x94TMxe2x80x94B based permanent magnets are made up of at least two phases, that is a major phase of a single crystal of an R2TM14B intermetallic compound and a grain boundary phase, such as an R3TM intermetallic compound phase.
The presence of the grain boundary phase is indispensable for the demonstration of coercivity of a magnet. Generally, the coercivity decreases as the R component necessary for forming the boundary phase becomes short. The reason is possibly that the two phases, namely the R2TM14B phase and the R3TM phase cease to be able to co-exist in the equilibrium state due to shortage of the R component and that, in its stead, the ferromagnetic phase such as R2TM17 phase is precipitated in the grain boundary of the R2TM14B phase to form an origin of generation of the inverse magnetic domain to produce inversion of magnetization easily to lead to lowered coercivity.
The presence of the grain boundary phase is indispensable for the demonstration of practically useful coercivity. The reason is possibly that the two phases, namely the R2TM14B phase and the Rxe2x80x94TM phase cease to be able to co-exist in the equilibrium state due to shortage of the R component and that, in its stead, the ferromagnetic phase such as R2TM17 phase is precipitated into the grain boundary of the R2TM14B phase to form an origin of generation of the inverse magnetic domain to produce inversion of magnetization easily to lead to lowered coercivity. The region of the composition in which the above-mentioned R2TM14B phase and the Rxe2x80x94TM phase coexist may be known from the Rxe2x80x94Fexe2x80x94B ternary equilibrium diagram.
The present inventors have found that there exists the following problem in displaying superior magnetic properties proper to the Rxe2x80x94TMxe2x80x94B based permanent magnet of the aforementioned prior art. That is, although the information on the composition range where there exists the R3TM grain boundary phase or on the possible presence of the defects in the interface between the major phase and the grain boundary phase has been acquired in the prior art, there lacked the knowledge as to the crystal structure or the R3TM grain boundary phase or the desirable relative orientation with respect to the major phase. Therefore, it has not been possible to control the microscopic structure of the Rxe2x80x94TMxe2x80x94B based permanent magnet having the specified composition to display superior magnetic properties. Instead, the various conditions of the magnet manufacturing process are optimized in the prior art with a view to empirically improving magnetic properties of the magnet.
That is, the magnetic properties of the magnet, in particular the structure of the interface between the major phase governing the coercivity and the grain boundary phase, were not known in the prior art. Thus, a variety of processing operations felt to vary the interface structure, such as heat treatment, are performed on the magnet to control the properties of the magnet, with the interface state remaining as a black box. Although this technique is not obstructive to the optimization of the manufacturing conditions of the magnets of various compositions, it is extremely difficult to improve the properties of the magnet further in the absence of the material development guideline as to what should be the ideal interface structure.
The present inventors have conducted microscopic analyses of the grain boundary phase of a variety of Rxe2x80x94TMxe2x80x94B based permanent magnets, using a transmission electron microscope (TEM), and found that, in the grain boundaries of all Co-containing Rxe2x80x94TMxe2x80x94B based permanent magnets, there necessarily exists a grain boundary phase composed of a R3TM intermetallic compound having a rhombic crystal system, with Co in TM of a R3TM being not less than 90 at %, and that superior magnetic properties can be realized when the major face contacts the grain boundary phase interposed with an interface.
The present inventors also conducted detailed scrutiny into the structure of the interface between the grain boundary phase of the Rxe2x80x94TMxe2x80x94B based permanent magnet having the R3TM grain boundary phase of the above-mentioned rhombic structure and the major phase (R2TM14B phase) by observation over a high resolution transmission electron microscope (HR-TEM) or a scanning tunnel microscope, and found that the magnetic properties are optimum when the microscopic structure of the permanent magnet is controlled so that the major phase and the grain boundary phase will have a specified relative crystallographic orientation in the vicinity of the interface to be matched with each other.
In its first aspect of the forth group, the present invention provides an Rxe2x80x94TMxe2x80x94B based permanent magnet composed of a magnetic phase mainly containing an R2TM14B intermetallic compound having a tetragonal crystal structure (R: rare earth element including Y; TM: transition metal) and a grain boundary phase containing an Rxe2x80x94TMxe2x80x94O compound, wherein the crystal structure of the grain boundary phase in the vicinity of an interface between the magnetic phase and the grain boundary phase is of face-centered cubic structure, and wherein the grain boundary phase is matched with the magnetic phase.
In the second aspect of the forth group, the Rxe2x80x94TMxe2x80x94O compound is precipitated in the vicinity of the interface in the grain boundary phase. In the third aspect of the forth group, in the R2TM14B intermetallic compound, the sum of Nd and/or Pr in R is not less than 50 at %, TM is Fe and/or Co, and Fe in TM is not less than 50 at % and, in the Rxe2x80x94TMxe2x80x94O compound, the ratio of R to the sum of R and TM is not less than 90 at %, the ratio of 0 is not less than 1 at % and not larger than 70 at %. In the fourth aspect of the forth group, the crystallographic orientation in the vicinity of an interface between the magnetic phase and the grain boundary phase is represented by at least a set of expressions (A) to (C):
(001)magnetic phase//(110)grain boundary phase and [110]magnetic phase//[001]grain boundary phasexe2x80x83xe2x80x83(A)
(001)magnetic phase//(221)grain boundary phase and [110]magnetic phase//[111{overscore ( )}]grain boundary phasexe2x80x83xe2x80x83(B)
(001)magnetic phase//(111)grain boundary phase and [100]magnetic phase//[11{overscore ( )}0]grain boundary phasexe2x80x83xe2x80x83(C)
wherein the angle of deviation in the crystallographic orientation is less than 5xc2x0.
In its fifth aspect of the second group, the permanent magnet is composed that
R is 8 to 30 at %;
B is 2 to 40 at %; with
Fe is 40 to 90 at %; and
Co is 50 at % or less.
In the sixth aspect of the forth group, the permanent magnets contains a magnetic phase having a tetragonal system and a grain boundary phase in which there exists an oxygen-containing crystal structure having a face-centered cubic structure in the vicinity of an interface to the magnetic phase, the magnetic phase matching with the grain boundary phase with the interface in-between.
In its seventh aspect of the forth group, the present invention includes precipitating an R2TM14B tetragonal crystal phase from an alloy containing R (rare earth element including Y), TM (transition metals), B and O and precipitating an Rxe2x80x94TMxe2x80x94O face-centered cubic structure around the R2TM14B tetragonal crystal phase such as to match the Rxe2x80x94TMxe2x80x94O face-centered cubic structure to the R2TM14B tetragonal crystal phase to elevate magnetocrystalline anisotropy of the R2TM14B tetragonal crystal phase in the vicinity of the epitaxial interface. Preferably, a source of an R2TM14B intermetallic compound exhibiting ferromagnetism (R: rare earth element including Y, and TM is a transition metal) and a source of the Rxe2x80x94TMxe2x80x94O compound is used as a starting material.
Taking an example of an Rxe2x80x94TMxe2x80x94B based permanent magnet, composed of the major phase (ferromagnetic phase) mainly composed of an R2TM14B intermetallic compound (preferably single crystal) and the grain boundary phase composed of an Rxe2x80x94TMxe2x80x94O compound, the principle in the fourth group of the present invention is explained. In a known manner, there exist in the Rxe2x80x94TMxe2x80x94B based permanent magnet a B-rich phase (R1+xcex1TM4B4), an Rxe2x80x94TM meta-stable phase, and oxides and carbides, in addition to the aforementioned major phase and the grain boundary phase. However, the effects of these phases on the magnetic properties of the permanent magnet are of subsidiary nature.
The presence of the grain boundary phase is indispensable for the demonstration of practically useful coercivity. Generally, the coercivity decreases as the R component in the magnet composition necessary for forming the grain boundary phase becomes short. The reason is possibly that the two phases, namely the R2TM14B phase and the Rxe2x80x94TM phase cease to be able to co-exist in the equilibrium state due to shortage of the R component and that, in its stead, the ferromagnetic phase such as R2TM17 phase is precipitated into the grain boundary of the R2TM14B phase to form an origin of generation of the inverse magnetic domain to produce inversion of magnetization easily to lead to lowered coercivity. The region of the composition in which the above-mentioned R2TM14B phase and the Rxe2x80x94TM phase coexist may be known from the Rxe2x80x94Fexe2x80x94B ternary equilibrium diagram.
For affording practically sufficient coercivity to the Rxe2x80x94TMxe2x80x94B based permanent magnet, prepared by the sintering method, it has been found necessary that the major phase as the ferromagnetic phase be contacted with the grain boundary phase at a smooth interface free of lattice defects, as has been clarified by microscopic observation of the interface over a transmission electron microscope. The reason is that, if there is a lattice defect in the interface, this lattice defect becomes the source of generation of the reverse magnetic domain to induce inversion of magnetization easily to lower the coercivity.
The present inventors have found that there exists the following problem in displaying superior magnetic properties proper to the Rxe2x80x94TMxe2x80x94B based permanent magnet of the above-mentioned prior art. That is, although the information on the composition range where there exists the Rxe2x80x94TM grain boundary phase or on the possible presence of the defects in the interface between the major phase and the grain boundary phase has been acquired in the prior art, there lacked the knowledge as to the crystal structure or the Rxe2x80x94TM grain boundary phase or the desirable relative orientation with respect to the major phase. Therefore, it has not been possible to control the microscopic structure f the Rxe2x80x94TMxe2x80x94B based permanent magnet having the specified composition to display superior magnetic properties. Instead, the various conditions of the magnet manufacturing process are optimized in the prior art with a view to empirically improving magnetic properties of the magnet.
The present inventors also conducted detailed scrutiny into the structure of the interface between the grain boundary phase of the Rxe2x80x94TMxe2x80x94B based permanent magnet having the Rxe2x80x94TM grain boundary phase of the above-mentioned face-centered cubic structure and the major phase (R2TM14B phase) by observation over a high resolution transmission electron microscope (HR-TEM) or a scanning tunnel microscope, and found that the magnetic properties are optimum when the microscopic structure of the permanent magnet is controlled so that the major phase and the grain boundary phase will have a specified relative crystallographic orientation in the vicinity of the interface to be matched with each other. The present invention has been brought to completion on the basis of this finding and our further perseverant researches.
The present inventors have conducted microscopic analyses on the grain boundary phase of a variety of Rxe2x80x94TMxe2x80x94B based permanent magnets, using a transmission electron microscope (TEM), and found that, in the grain boundaries of Rxe2x80x94TMxe2x80x94B based permanent magnets, and that superior magnetic properties can be realized, if there exists a grain boundary phase composed of a Rxe2x80x94TMxe2x80x94O alloy containing not less than 90 at %, and the crystal structure of a portion of the grain boundary phase in the vicinity of the interface relative to the major phase has a face-centered cubic structure.
The present inventors also conducted detailed scrutiny into the structure of the interface between the grain boundary phase of the Rxe2x80x94TMxe2x80x94B based permanent magnet having the Rxe2x80x94TMxe2x80x94O grain boundary phase of the above-mentioned face-centered cubic structure and the major phase (R2TM14B phase) by observation over a high resolution transmission electron microscope (HR-TEM) or a scanning tunnel microscope, and found that the magnetic properties are optimum when the microscopic structure of the permanent magnet is controlled so that the major phase and the grain boundary phase will have a specified relative crystallographic orientation in the vicinity of the interface. The present invention has been brought to completion on the basis of this finding and our further perseverant researches.
In the first aspect of the fifth group of this present invention, the present invention provides rare-earth magnetic powders for bonded magnets wherein alkaline earth metals exist in an interface of an R2TM14B phase (R: rare earth element including Y and TM is a transition metal) in a epitaxial state relative to the R2TM14B phase.
In the other aspect of the fifth group of this present invention, the present invention provides rare-earth magnetic powders for bonded magnets wherein the crystallographic orientation in the vicinity of an interface between the magnetic phase and said alkaline earth metal phase is represented by at least a set of expressions (A) to (E):
(001)major phase//(110)grain boundary phase and [110]major phase//[001]grain boundary phasexe2x80x83xe2x80x83(A)
(001)major phase//(221)grain boundary phase and [110]major phase//[111{overscore ( )}]grain boundary phasexe2x80x83xe2x80x83(B)
(001)major phase//(111)grain boundary phase and [100]major phase//[11{overscore ( )}0]grain boundary phasexe2x80x83xe2x80x83(C)
(001)major phase//(201)grain boundary phase and [110]major phase//[010]grain boundary phasexe2x80x83xe2x80x83(D)
(001)major phase//(22{overscore ( )}3)grain boundary phase and [110]major phase//[110]grain boundary phasexe2x80x83xe2x80x83(E).
In the further aspect of the fifth group of this present invention, the present invention provides a method for producing rare-earth magnetic powders for bonded magnets including the steps of impregnating alkaline earth metal in powders mainly composed of magnetic powders containing the R2TM14B phase (R: rare earth element including Y, and TM: transition metal).
In the present specification, the statement xe2x80x9calkaline earth metal existsxe2x80x9d means not only a case in which an alkaline earth metal exists by itself, but also a case in which it exists as an alloy, a compound or a mixed state thereof.
The present inventors have found that, if an Nd2+xFe14B compound (x=0.0 to 0.2) is dissolved, the ingot is pulverized to a pre-set particle size and Ca metal is impregnated into the powders from the particle surface, coercivity can be improved significantly as compared to the case where Nd metal is impregnated. The present invention has been completed on the basis of this finding and on our further researches.
According to the fifth group of the present invention, it is possible to provide high coercivity magnetic powders of R2TM14B based rare earth elements directly exploiting features of the nucleation type rare earth element without forcibly pulverizing the nucleation type rare earth element magnetic powders into a pinning type rare earth element magnetic powders having a reduced crystal grain size. In addition, since the production process of the magnetic powders of R2TM14B based rare earth elements is simplified, the production costs are lowered and the product quality is stabilized.
Referring to FIGS. 1 and 2A and 2B, the difference between the distribution of magnetocrystalline anisotropy in the neighborhood of the interface with the major phase (or ferromagnetic phase) matching to the grain boundary phase (such as Rxe2x80x94TM, R3TM, Rxe2x80x94TMxe2x80x94O and Ca metals) and that with the major phase (or ferromagnetic phase) mismatching to the grain boundary phase is explained. In FIGS. 1 and 2A and 2B, the xe2x80x9coutermost shellxe2x80x9d denotes the position of an outermost atomic layer of the major phase, while the xe2x80x9csecond layerxe2x80x9d and the xe2x80x9cthird layerxe2x80x9d denote second and third atomic layers as counted from the outermost shell position towards the inside, respectively. The nth layer denotes a position remote from the outermost shell such that the effect from the interface is negligible. In the graph of FIG. 1, the ordinate denotes the intensity of the uniaxial magnetic anisotropy constant K1 representing the intensity of the magnetocrystalline anisotropy. The larger the value of K1, the more the orientation of the major phase is stabilized in the direction of easy axis (c-axis direction). Also, in FIG. 1, the Example (inventive) shows calculated values of K1 under the condition of the major phase and the grain boundary phase matching with each other on the interface, as shown in FIG. 2A, while the Comparative Example shows the calculated value of K1 when the interface mismatching exists due to dropout of the grain boundary phase or the like as shown in FIG. 2B.
Referring to FIG. 1, the magnitude of the anisotropic constant K1 varies significantly in the Comparative Example with the distance from the interface, with the value of K1 in the outermost shell being significantly lowered from the value in the interior. In the Example, the magnitude of the anisotropic constant K1 is not significantly changed with the distance from the interface. Rather, the anisotropic constant K1 is increased in the outermost shell phase. Therefore, in the Comparative Example, the energy required for nucleation of the inverse magnetic domain (demagnetizing field) is locally lowered to facilitate nucleation and inversion of magnetization, thus lowering the coercivity of the magnet. In the Example, K1 in the outermost shell is somehow higher than that in the interior, thus suppressing nucleation of the inverse magnetic domain in the interface to increase coercivity of the magnet.
The meritorious effect of the present invention are summarized as follows.
The present invention provides a guideline for designing permanent magnets having high magnetic performance, in particular coercivity. Up to now, the structure of the interface between the major phase and the grain boundary phase responsible for coercivity was not known. Since the ideal interface structure for improving the coercivity has been clarified by the present invention, a new guideline for developing permanent magnets is provided, while the pre-existing permanent magnet (particularly, Rxe2x80x94TMxe2x80x94B based one) can be improved further in coercivity. The result is that novel permanent magnet materials can be found easily, while permanent magnet (particularly, Rxe2x80x94TMxe2x80x94B based one), so far not used practically because of the low coercivity, can be put to practical use, and an optimum composition can be determined easily.
With the Rxe2x80x94TMxe2x80x94B based permanent magnet according to the present invention, the relative position between atoms in the interface between the major and grain boundary phases is regular and matched with each other, thereby decreasing the possibility of the interface operating as an originating point of the inverse magnetic domain (demagnetizing field) to achieve high coercivity. Also, the Rxe2x80x94TMxe2x80x94B based permanent magnet according to the present invention has superior magnetic properties since specified crystal orientation between the ferromagnetic phase and the grain boundary phase strengthens the crystal field of the R atom in the major phase in the vicinity of the interface to raise the magnetocrystalline anisotropy in the vicinity of the interface of the major phase so that the inverse magnetic domain in the vicinity of the grain boundary can hardly be produced to render facilitated inversion of magnetization difficult.
The magnetic powders of the rare earth element for bonded magnets, obtained with the present invention, are superior in magnetic properties as compared to those obtained with the conventional rapid solidification method or HDDR method and can be manufactured by a simpler method. Therefore, by applying the powders of the present invention, the rare earth element bonded magnets can be produced at a lower cost to provide inexpensive rare earth element bonded magnets with high magnetic properties. The inventive powders are particularly useful as the magnetic powders for high coercivity materials. In the midst of a demand for magnet size reduction, the present invention provides a technique useful for improving coercivity of the ultra-small-sized Nd2TM14 B based magnet.